The relatively recently developed science of grain boundary engineering is based upon the well-documented fact that different structures can exist for different grain boundaries in metals. It has been shown that the structural differences can lead to different grain boundary properties, such as their energy.
The concept of a coincident site lattice (CSL), whereby at certain crystallographic misorientations, a three-dimensional lattice can be constructed with lattice points common to both adjacent crystals, is also extremely important. The CSL is considered to the smallest common sub-lattice of the adjoining grains. The volume ratio of the unit cell of the CSL to that of the crystal is described by the parameter .SIGMA., which can also be considered the reciprocal density of coincidental sites. All grain boundaries can be represented by an appropriate CSL description if .SIGMA. is allowed to approach infinite values.
Grain boundary engineering finds application in four main areas: (a) ordered intermetallic compounds where it can be used to control composition and state of order at grain boundaries and control the distribution of misorientation of neighbouring grains. (b) Electrical properties of materials as studies have shown that electrical resistivity of grain boundaries varies with misorientation of adjacent crystals and is at a minimum for CSL or twin boundaries. (c) Nanocrystalline materials and the production of triple junctions therein and (d) materials for nuclear waste storage by inhibition of crevice corrosion and hydrogen induced cracking in titanium alloys and uniform corrosion in copper alloys.
In mechanical, superconducting and semiconducting applications the use of polycrystalline materials is attractive as they are relatively inexpensive and are easy to produce. However, limitations arise due to the properties of different types of grain boundaries and the anisotropic nature of the individual grains. For example, cracks can propagate easily along certain grain boundaries and cleave grains oriented in certain directions. In orthorhombic YBa.sub.2 Cu.sub.3 O (YBCO) high temperature superconductors, the current density is higher by a factor of five to ten in the basal plane than in the c-direction. It has also been shown that the critical current density at the boundary decreases with increasing misorientation angles in such materials. In polysilicon, lower electron mobilities have been a problem. A large number of investigations on single grain boundaries in bi-crystals and in polysilicon have demonstrated their electrical activity as recombination centers. Small angle grain boundaries (.theta.&lt;10.degree.) show an efficient current degradation (20-30%), while random large angle grain boundaries are usually strong recombination centers. Coherent twins or other low energy near-coincidence site lattice (CSL) boundaries are not at all or very weakly active.
Other than mobility problems with semiconducting materials, as device dimensions approach deep sub-micron sizes, the reliability of the interconnect in terms of electromigration and also the interconnect resistance becomes increasingly important. Therefore, lower resistance metals and the ways to reduce their resistance further in thin film form are being investigated. Copper thin films are attractive as ultra large scale integration (ULSI) conductor materials as they have a lower resistivity and higher melting point as compared to the presently employed aluminum films. Reducing the resistivity further by employing textured films to enhance mobility would allow interconnect dimensions to shrink without compromising electromigration resistance.
For mechanical materials, texture is usually controlled by deformation and post-deformation controlled recrystallization. For bulk superconductors, melt-textured growth has been applied to control the texture. Textured thin films are usually obtained by a combination of deposition method and choice of substrate material.
A zone confining process for the production of tin-doped indium oxide (ITO), thin films having chains of grains oriented in the same direction has been described by the present inventor in Journal of Materials Science Letters 12:1902 (1993), the disclosure of which is incorporated herein by reference. There remains a need to extend the work on ITO to other alloys and metals and to improve, by grain boundary engineering techniques, the physical and electrical properties of vapour deposited and other films and to bulk planar polycrystalline materials up to about 1 cm thick.